1. Field of the Invention
The present invention relates generally to an actuator positioning system in a direct access storage device (DASD) and more particularly to a phase modulated servo positioning method and apparatus for use with narrow transducer heads in a disk file.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
All DASD units must have a method to position each data head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level files using a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the heads on a given track. Typically, track accessing and track following are provided utilizing a magnetically written pattern in the DASD unit. A dedicated servo system employs one surface of one of the disks in the DASD on which to have all the tracking and access information. A sector servo system uses small portions of tracks between each or between several sectors on each track of each data surface to provide the tracking and access information. A hybrid servo system uses both to obtain advantages of each type of servo.
Magneto-resistive (MR) heads are a great advancement in read/write technology for DASD. One of the key advantages of MR heads is the use of separate elements for reading and writing. This allows the writing element to be wide, almost a full track pitch in width, and the reading element to be narrow. This write wide, read narrow capability eases the track misregistration (TMR) requirements on the servo system by effectively increasing the width of the soft-error rate (SER) bathtubs by the difference in widths between the read and write elements.
Servo tracks are typically written on a 1/2 track pitch. A traditional phase pattern is written so that the phase of each burst changes 22.5 degrees for each written servo track. For increasing track types, the phase is advanced in bursts 1 and 3 and retarded in burst 2. In this fashion, the phase between burst 1 and burst 2 changes 45 degrees for each servo track written.
This means that a 5 Mhz pattern can be written with a 80 Mhz clock. In this case the 5 Mhz period consists of 16, 80 Mhz clocks and the phase of each burst is changed by one bit for each servo track written. Then the difference in phase between the bursts repeats in eight servo tracks. The eight servo tracks written on a 1/2 track pitch correspond to four data tracks or cylinders.
To optimize the SER, a reading transducer width that is less than 50% of the track pitch is required. With reading elements less than the width of a single servo track, position linearity becomes unacceptably poor with the conventional 1/2 track pitch servo tracks. Linearity error can be improved by writing narrower tracks. For example, the phase pattern written on a 1/4 track pitch would provide acceptable position linearity; however, this would double the time required for servo writing and would be prohibitively expensive for a sector servo file.
If a conventional servo pattern is written on a narrow track pitch, for example, 3/8 track pitch, the position linearity requirements will be satisfied, but the pattern will repeat in three data cylinders instead of four. This means that the position error signal (PES) extends +/-1.5 tracks instead of +/-2 tracks before the PES repeats. When seeking, this 50% loss in range is significant. Seek velocities can be as high as 40 or 50 tracks/sample. This means that position must be estimated 50 tracks away, therefor the PES error must never be greater than 1.5 tracks out of 50, or 3%. If the error is greater than this amount, the estimator algorithm may converge on the wrong position and velocity and be off by modulo three tracks. As a result a conventional servo pattern with a narrow track pitch pattern is unacceptable.